Hydraulic roughness is the measure of the amount of frictional resistance water experiences when passing over land and
channel
Channel, channels, channeling, etc., may refer to:
Geography
* Channel (geography), in physical geography, a landform consisting of the outline (banks) of the path of a narrow body of water.
Australia
* Channel Country, region of outback Austral ...
features.
One roughness coefficient is ''
Manning's n-value''.
Manning’s n is used extensively around the world to predict the degree of roughness in channels.
Flow velocity
In continuum mechanics the flow velocity in fluid dynamics, also macroscopic velocity in statistical mechanics, or drift velocity in electromagnetism, is a vector field used to mathematically describe the motion of a continuum. The length of the f ...
is strongly dependent on the resistance to flow. An increase in this n value will cause a decrease in the velocity of water flowing across a surface.
Manning's n
The value of Manning’s n is affected by many variables. Factors like
suspended load, sediment
grain size, presence of
bedrock or boulders in the stream channel, variations in channel width and depth, and overall
sinuosity
Sinuosity, sinuosity index, or sinuosity coefficient of a continuously differentiable curve having at least one inflection point is the ratio of the curvilinear length (along the curve) and the Euclidean distance (straight line) between the en ...
of the stream channel can all affect Manning’s n value. Biological factors have the greatest overall effect on Manning’s n; bank stabilization by vegetation, height of grass and brush across a floodplain, and stumps and logs creating natural dams are the main observable influences.
Biological Importance
Recent studies have found a relationship between hydraulic roughness and salmon spawning habitat; “bed-surface grain size is responsive to hydraulic roughness caused by bank irregularities, bars, and wood debris… We find that wood debris plays an important role at our study sites, not only providing hydraulic roughness but also influencing pool spacing, frequency of textural patches, and the amplitude and wavelength of bank and bar topography and their consequent roughness. Channels with progressively greater hydraulic roughness have systematically finer bed surfaces, presumably due to reduced bed
shear stress
Shear stress, often denoted by (Greek: tau), is the component of stress coplanar with a material cross section. It arises from the shear force, the component of force vector parallel to the material cross section. ''Normal stress'', on the ...
, resulting in lower channel competence and diminished
bed load
The term bed load or bedload describes particles in a flowing fluid (usually water) that are transported along the stream bed. Bed load is complementary to suspended load and wash load.
Bed load moves by rolling, sliding, and/or saltating (h ...
transport capacity, both of which promote textural fining”. Textural fining of stream beds can effect more than just salmon spawning habitats, “bar and wood roughness create a greater variety of textural patches, offering a range of aquatic habitats that may promote biologic diversity or be of use to specific animals at different life stages.”
[Buffington, J., & Montgomery, D. (1999). Effects of hydraulic roughness on surface textures of gravel-bed rivers. Water Resources Research, 35(11), 3507-3521. Retrieved December 15, 2015, http://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.459.6346&rep=rep1&type=pdf]
References
Erosion
Hydraulics
Hydraulic engineering
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